The overall objective of this project is to determine the regional cerebral blood flow (rCBF) patterns in the rat following subrachnoid hemorrhage (SAH). SAH is induced in the middle cerebral artery by inserting a monofilament into the right internal carotid artery and to the junction with the MCA where it is used to cause an arterial rupture. Quantitative cerebral blood flow maps are obtained using the arterial spin labeling technique developed at this Center. rCBF maps are obtained before, and for four hours at half hour intervals beginning 30 min post hemorrhage. These results will be correlated with histopathological changes at 8 days post-insult. The time course of the ischemic changes in rCBF will also be correlated with changes in plasma biogenic amines (catecholamines and serotonin) which have been implicated in the exacerbation of ischemic injury after SAH. Previously, we had characterized the rCBF at time points 3- and 8- days postinjury. The current study clearly show that those regions previously showing infarction on days 3 and 8 after SAH, namely, the MCA distribution of the frontoparietal cortecies and in the lateral parietal cortex with some ischemic neuronal changes in the hippocampal areas show similar behaviour within the first 4 hours after SAH. Thus future characterizations of the SAH model will be carried out within the first 24 hours after the insult, rather than 3 or 8 days postinjury as we have done in the past.